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. . 2 ",",-=--.....1 ~,~~-:-..-..t-~ c.~'f" 1 ORn !\l..;~".."'" !:.'~,.. C'....~, :t'l'"'l.,., "'-'-Y"0~~ SLltt.k"1.S and GLJJ....... J.J..J...L.1\,.t"'~-L.'-J .L./.......v, ~...-"-...A"""....--.....J -v ~...... _ ~...._--, Clemmer 1979), that the Little Colorado River is an important spawning and nursery area for this endangered species. Additionally, as this system was in flood during April, 1979, with flows exceeding 3000 cfs, it is apparent that after fl'ood waters had receeded, that this species spavmed in this area. This use reflects the extended spawning period for humpback chubs, which ran~es from March through mid-summer (Minckley et al.,in press), and the resilience of native fishes in their response to less than favorable conditions ~within their habitat. Age Composition ---- Age information on the humpback chub is limited however, based on data presented by Carothers and Minckley (1980), more than ten age groups were represented ~in July collections. In general, fish ranged from 86-92 rom during their first year of life and reached 358 mm by their ninth year of life (Carothers and Minckley 1980). Older fish dominated the July collection with 26.4 % being nine years of age or older. The second most common age class was four-year old fish (14.9%) followed by young- of the year fish, which represented 13.8% of the collection. Relative Density and Relative 'Abundance ---~ The relative density of young-of-the-year and juvenile humpback chubs occurrin& within the Little Colorado River, varied from 582 fish/hectare, .4 km upstream, to 407 f/ha at the con- fluence. These values are lower than the 1978 summer es- timates made at the upstream locality (1063 f/ha) and higher than the 1978 confluent estimate (278 f/ha), but do fall within previous estimates of 167-1063 f/ha for those areas (Carothers and Minckley 1980). The relative density of larger fish connot be readily addressed. Based on collection data, it is apparent that they were abundant, but no estimate can be made of the numbers present. Based on previous data (Carothers and Minckley 1980, Minckley 1977, ;, it is obvious that the confluent area of the Little Colorado River is heavily used during the spring and summer, and decreases during the fall and winter. The reason(s) for this wide variation in the numbers of smaller fish and adults is not apparent, but is probably rel'ated to reproductive success, strea~ flow in the Little Colorado River, both natural and as impacted by Glen Canyon Dam operation, and by possible food availability. As with other life history parameters of this species, further research is necessary to document what factors control habitat use by the humpback chub in the Little Colorado River.